The present invention relates to lamps. More particularly, the present invention relates to multi-filament lamps.
Inside the glass envelope or bulb of an incandescent lamp, there is a filament extending between two terminals. When a voltage is applied across the terminals, the resistance of electric current running through the filament causes the filament to heat to the point where it will glow.
Eventually, the operation of the lamp including thermal expansion and contraction of the filament when the light is turned on and off, will cause the filament to fail or xe2x80x9cburnoutxe2x80x9d. When a lamp burns out, it must be replaced. The burned-out lamps are not repaired.
Some bulbs contain more than one filament. These bulbs will give off different light intensities depending on how many of the filaments are conducting current. Nonetheless, when all of the filament fails, the bulb will not light.
Light bulbs will last for several hundred hours before burning out. Some light bulbs will last much longer and are referred to as xe2x80x9clong lifexe2x80x9d bulbs. Long life bulbs are made with heavier gauge filaments. However, when the filament fails, the bulb must be replaced.
Thus, there remains a need for a way to extend the life of a light bulb.
According to its major aspects and briefly recited, the present invention is the bulb having at least two filaments wherein, when the first filament burns out, current will be conducted across the electrodes in the bulb by the next filament. To turn on the second filament, a shunt, made of a fine wire, is closed to connect the second filament between the secondary filament electrode of the circuit and the common electrode, thus relighting the lamp. The open circuit voltage welds the shunt to the primary and secondary electrodes.
Each additional filament has its own electrode and shunt that will link its electrode back to the primary filament electrode. Each shunt will have a different resistance. When the primary filament burns out, an arbitration takes place among the remaining filaments in order of resistance of their shunts. The one with the first conductance welds its shunt closed.
In addition, when a bulb according to the present preferred invention is used with other bulbs in a series circuit, such as with Christmas light strings, an additional, bypass shunt can be placed in each bulb between the second filament electrode and the common electrode. After the secondary filament burns out, this shunt will weld itself across the electrodes and conduct so that the light string remains on notwithstanding the fact that a bulb has burned out.
A feature of the present invention is the use of at least one backup filament. When the first filament burns out, the light will operate using the second filament. Not only does the second filament extend the useful life of the light by about a factor of two, but also it reduces the time and cost of changing light bulbs by the same factor. Even allowing for somewhat higher manufacturing cost of the present dual, sequential lamp, the overall cost savings of the present lamp compared to prior art lamps is significant.
Another feature of the present invention is a switch inside the bulb that operates when the first filament burns out. Therefore, the present bulb can be used in current light sockets without modification to the lamp socket because there is no change in the exterior configuration of the bulb.
Still another feature of the present invention is the use of the open circuit voltage and a shunt to switch the current to the second filament. This feature enables the second filament to operate sequentially and automatically on the failure of the first filament.
In an alternative embodiment, as part of a plurality of lights in a series circuit, use of a bypass shunt to pass current when all of the filaments are burned out is an important feature of this embodiment of the present invention.